Gate tunability of stray-field-induced electron spin precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe grating

Time-resolved Faraday rotation is used to measure the coherent electron spin precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe grating. We show that the electron spin precession frequency can be modified by applying a gate voltage of opposite polarity to neighboring bars. A tunability of the precession frequency of 0.5 GHz/V has been observed. Modulating the gate potential with a gigahertz frequency allows the electron spin precession to be controlled on a nanosecond timescale

Optimized stray-field-induced enhancement of the electron spin precession by buried Fe gates

The magnetic stray field from Fe gates is used to modify the spin precession frequency of InGaAs/GaAs quantum-well electrons in an external magnetic field. By using an etching process to position the gates directly in the plane of the quantum well, the stray-field influence on the spin precession increases significantly compared with results from previous studies with top-gated structures. In line with numerical simulations, the stray-field-induced precession frequency increases as the gap between the ferromagnetic gates is reduced. The inhomogeneous stray field leads to additional spin dephasing.Comment: 4 pages, 2 figure

Two-dimensional imaging of the spin-orbit effective magnetic field

We report on spatially resolved measurements of the spin-orbit effective magnetic field in a GaAs/InGaAs quantum-well. Biased gate electrodes lead to an electric-field distribution in which the quantum-well electrons move according to the local orientation and magnitude of the electric field. This motion induces Rashba and Dresselhaus effective magnetic fields. The projection of the sum of these fields onto an external magnetic field is monitored locally by measuring the electron spin-precession frequency using time-resolved Faraday rotation. A comparison with simulations shows good agreement with the experimental data.Comment: 6 pages, 4 figure

Mode Spectroscopy and Level Coupling in Ballistic Electron Waveguides

A tunable quantum point contact with modes occupied in both transverse directions is studied by magnetotransport experiments. We use conductance quantization of the one-dimensional subbands as a tool to determine the mode spectrum. A magnetic field applied along the direction of the current flow couples the modes. This can be described by an extension of the Darwin-Fock model. Anticrossings are observed as a function of the magnetic field, but not for zero field or perpendicular field directions, indicating coupling of the subbands due to nonparabolicity in the electrical confinement.Comment: 4 pages, 3 figure

Spin-orbit interaction and spin relaxation in a two-dimensional electron gas

Using time-resolved Faraday rotation, the drift-induced spin-orbit Field of a two-dimensional electron gas in an InGaAs quantum well is measured. Including measurements of the electron mobility, the Dresselhaus and Rashba coefficients are determined as a function of temperature between 10 and 80 K. By comparing the relative size of these terms with a measured in-plane anisotropy of the spin dephasing rate, the D'yakonv-Perel' contribution to spin dephasing is estimated. The measured dephasing rate is significantly larger than this, which can only partially be explained by an inhomogeneous g-factor.Comment: 6 pages, 5 figure

Variation of elastic scattering across a quantum well

The Drude scattering times of electrons in two subbands of a parabolic quantum well have been studied at constant electron sheet density and different positions of the electron distribution along the growth direction. The scattering times obtained by magnetotransport measurements decrease as the electrons are displaced towards the well edges, although the lowest-subband density increases. By comparing the measurements with calculations of the scattering times of a two-subband system, new information on the location of the relevant scatterers and the anisotropy of intersubband scattering is obtained. It is found that the scattering time of electrons in the lower subband depends sensitively on the position of the scatterers, which also explains the measured dependence of the scattering on the carrier density. The measurements indicate segregation of scatterers from the substrate side towards the quantum well during growth.Comment: 4 pages, 4 figure

Long-Lived Spin Coherence States

We study evolution of electron spin coherence having non-homogeneous direction of spin polarization vector in semiconductor heterostructures. It is found that the electron spin relaxation time due to the D'yakonov-Perel' relaxation mechanism essentially depends on the initial spin polarization distribution. This effect has its origin in the coherent spin precession of electrons diffusing in the same direction. We predict a long spin relaxation time of a novel structure: a spin coherence standing wave and discuss its experimental realization

Cenários de desenvolvimento sustentável no Pantanal em função de tendências hidroclimáticas.

Nesta feita uma análise prospectiva do futuro hidrológico do Pantanal sob a perspectiva de dados pretéritos, da mudança do clima com base em relações entre o nível máximo do Rio Paraguai, interação oceano-atmosfera e precipitação na América do Sul. A cenarização hidroclimática, incluindo a componente humana, permite ainda identificar adaptações e oportunidades para as próximas décadas, também em função de mudanças e tendências da economia mundial.bitstream/CPAP-2009-09/56873/1/DOC98.pd

MHz Unidirectional Rotation of Molecular Rotary Motors

A combination of cryogenic UV-vis and CD spectroscopy and transient absorption spectroscopy at ambient temperature is used to study a new class of unidirectional rotary molecular motors. Stabilization of unstable intermediates is achieved below 95 K in propane solution for the structure with the fastest rotation rate, and below this temperature measurements on the rate limiting step in the rotation cycle can be performed to obtain activation parameters. The results are compared to measurements at ambient temperature using transient absorption spectroscopy, which show that behavior of these motors is similar over the full temperature range investigated, thereby allowing a maximum rotation rate of 3 MHz at room temperature under suitable irradiation conditions